OFDM (Orthogonal Frequency Division Multiplexing, orthogonal frequency division multiplexing) belongs to FDM (Frequency Division Multiplexing, frequency division multiplexing) technologies. In an OFDM system, in order to maximize the spectrum efficiency, sub-carriers have overlapping portions. Generally, the adjacent channels with the overlapping portions may interfere with each other. However, in the OFDM system, the sub-carriers are precisely orthogonal with each other, and a maximum value point of the power of each sub-carrier directly corresponds to a minimum value point of the power of the adjacent channel, so the sub-carriers can partially overlap each other without mutual interferences. Therefore, through the OFDM system, the interference between the adjacent channels is avoided, and meanwhile, the spectrum efficiency is maximized. Since a channel transmission rate is increased with the increase of a channel bandwidth, compared with a common FDM system, the OFDM system allows higher data throughput, and the spectrum is more effectively used.
The OFDM system uses the digital signal processing technologies, and generation and receiving of each sub-carrier are completed through a digital signal processing algorithm, including IFFT (Inverse Fast Fourier Transform, inverse fast Fourier transform) and FFT (Fast Fourier Transform, fast Fourier transform), which greatly simplifies the system structure. Meanwhile, in order to improve spectrum utilization ratio, the spectrums on the sub-carriers overlap each other, and the spectrums meet orthogonality in a whole symbol period, which ensures that a receiving end may recover signals without distortion.
A baseband signal u(t) is defined, and an expression thereof is as follows:
                    S        B            ⁡              (        t        )              =          Re      ⁡              [                  u          ⁡                      (            t            )                          ]                                u        ⁡                  (          t          )                    =                        ∑                      n            =            0                                N            -            1                          ⁢                                  ⁢                              d            n                    ·                      ⅇ                          j2π              ⁢                                                          ⁢                              nf                0                            ⁢              t                                            ,                  ⁢                            d          n                =                              a            n                    +                      jb            n                              ;      
N sampling is implemented in a period T, and the following is obtained:
                    u        ⁡                  (                      k                          Nf              0                                )                    =                                    ∑                          n              =              0                                      N              -              1                                ⁢                                          ⁢                                    d              n                        ·                          ⅇ                              j2π                ⁢                                                                  ⁢                                  nf                  0                                ⁢                                  k                                      Nf                    0                                                                                      =                                            ∑                              n                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          d                n                            ·                              ⅇ                                  j                  ⁢                                                            2                      ⁢                      π                      ⁢                                                                                          ⁢                      nk                                        N                                                                                =                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          d                n                            ·                                                (                                      ⅇ                                          j                      ⁢                                                                        2                          ⁢                          π                                                N                                                                              )                                nk                                                          ,                  ⁢          (                        k          =          0                ,        1        ,        2        ,        …        ⁢                                  ,                  N          -          1                    )                  u      ⁡              (        k        )              =                  I        ⁢                                  ⁢        F        ⁢                                  ⁢        F        ⁢                                  ⁢                  T          ⁡                      (            dn            )                              =              I        ⁢                                  ⁢        F        ⁢                                  ⁢        F        ⁢                                  ⁢                              T            ⁡                          (                              an                +                jbn                            )                                .                    
The foregoing description is an OFDM modulation process, that is, an IFFT/IDFT (Inverse Discrete Fourier Transform, inverse discrete Fourier transform) algorithm process.
The OFDM modulation process is an FFT/DFT (Discrete Fourier Transform, discrete Fourier transform) process:
            u      ⁡              (        t        )              =                                        s            I                    ⁡                      (            t            )                          +                              js            Q                    ⁡                      (            t            )                              =                        ∑                      n            =            0                                N            -            1                          ⁢                                  ⁢                              d            n                    ·                      ⅇ                          j2π              ⁢                                                          ⁢                              nf                0                            ⁢              t                                                dn    =          F      ⁢                          ⁢      F      ⁢                          ⁢                        T          ⁡                      (                          u              ⁡                              (                k                )                                      )                          .            
The OFDM is the core technology of radio 4 G LTE (Long Term Evolution, Long Term Evolution), and applications of the OFDM to an optical network field is one of the technology hotspots in the industry. The OFDM is originally applied in a high-rate transmission system to improve the chromatic dispersion resistance of the system. Currently, applications of the OFDM technology in an optical network, or applications of the OFDM in a ring network, are also a latest technology hotspot.
In the prior art, a transmission network implemented by using an OFDMA (Orthogonal Frequency Division Multiplexing ACCESS, orthogonal frequency division multiplexing access) technology is provided. In the transmission network, the OFDMA technology and a ROADM (Reconfigurable Optical Add-drop Multiplexer, reconfigurable optical add-drop multiplexer) technology are integrated and applied in an access environment, and the ring network replaces a tree network of a current passive optical network. In a ring network, each network node allocates the loop bandwidth according to the number of the OFDM sub-carriers, and transmitting wavelengths of each node are G.692 wavelengths different from each other. All the nodes share a same OFDMA frame, and different wavelengths are superposed with each other. An optical receiver in the node is a multi-wavelength receiver, which receives all the wavelengths at the same time, and a tributary card of the receiver has functions required by the OFDMA, such as digital signal processing (IFFT/FFT), AD (analog-digital)/DA (digital-analog) conversion, M-QAM (M-Quadrature Amplitude Modulation, M-quadrature amplitude modulation) encoding/decoding, and port data traffic monitoring. A downlink OFDMA signal transmitted from a master node, OLT (Optical Line Terminal, optical line terminal), to a destination ONU (Optical Network Unit, optical network unit) in two loop directions reaches the ONU, and then reaches an 2×1 selective optical switch through a SPLITTER (splitter), and afterwards reaches an AD converter in the ONU for being converted into a digital signal, and then a data signal is restored through digital down conversion, FFT processing and M-QAM decoding processes. A downlink processing process of the ONU requires authorization of the OLT, and then the allocated OFDMA sub-carriers can be DROP DWON (dropped down). An uplink of the ONU initiates a bandwidth request to the OLT by monitoring the data traffic in a buffer, and the OLT calculates an allocation result through a bandwidth allocation algorithm after receiving all the bandwidth requests of the ONU, and then transmits a bandwidth authorization signal to the ONUs.
A frame structure on the ring network is a TDD (Time Division Duplex, time division duplex) manner, that is, the uplink and the downlink form a large frame which is divided into an uplink sub-frame and a downlink sub-frame on the time, and the uplink frame uses a two-dimensional frame structure manner of OFDMA and TDMA (Time Division Multiple Access, time division multiple access), and is divided into time slots with 125 μs as a unit. An uplink burst packet is loaded in each time slot, a format of which is similar to the format of the uplink burst packet in a GPON (Gigabit-Capable Passive Optical Network).
Another OTN (Optical Transport Network, optical transport network) technology is further provided in the prior art, and OTN-series recommendations, such as ITU-T G.709, G.798 and G.87X, formulated by the ITU-T (International Telecommunications Union-Telecommunications Standardization section, International Telecommunications Union-Telecommunications Standardization section) are already mature, and are commercially used in the OTN products in the industry. The main task of the OTN is to digitally encapsulate and transmit client signals with a rate higher than 1 G, the minimum cross scheduling granularity thereof is at a 1 G level, and the common cross scheduling granularity is at a 1 G/2.5 G/10 G level.
During the implementation of the present invention, the inventor finds that the prior art at least has the following problems.
1. Only all the convergence services from the slave node to the master node are supported, while private line connection between two slave nodes is not supported.
2. The channel granularity is rough, and the services with the rate higher than 1 GBIT/S may be scheduled, while the sub-rate services with the rate lower than 1 G cannot be flexibly scheduled. However, scheduling requirements based on the stream still exist on a backbone network, and the rates of the streams are variable and are lower than 1 G for example, an HDTV video stream or a 155 MBIT/S data stream.
3. The improvement is performed on the basis of a transmission technology system, and the application of the OFDM technology is limited on a WDM ring network, without consideration of the relation to and integration with the conventional transmission system.
4. Since the multiple wavelengths share one OFDMA frame, the loop capacity has no relation to the number of the wavelengths, and the capability of a large capacity of the multi-wavelength network cannot be reflected.